The present invention relates to a protective circuit according to the preamble of claim 1.
There have been known special vacuum cleaners or devices operating in the manner of a vacuum cleaner, which are capable also of taking up liquids. They are equipped for this purpose with a tank in which a vacuum is generated by means of a blower which is driven by a correspondingly strong electric motor so that it is possible to draw in and collect in this tank also liquids, via a corresponding line. There have also been known combined devices which are suited for drawing in dusts and solid matter, or the like, as well as water.
It is of course of problem of such devices, which will be described hereafter as liquid or water vacuum cleaners, that once the water level in the tank rises above a predetermined level, water particles or, quite generally, liquids may be drawn in which may be problematic for the function and trouble-free operation of such water vacuum cleaners, even when the corresponding shaft connections between the blower and the electric motor are sealed off sufficiently.
Consequently, it has been known before, and is common practice, to provide such water vacuum cleaners with level-controlled disconnection systems consisting, for example, of a water sensor arranged at a predetermined height inside the tank which may, for example, consist of two separate, current-carrying wires which, when liquid is drawn in, have resistance values differing in such a manner that this changed state can be detected and, for example, evaluated for switching off the motor.
It has also been known, in connection with other devices, to provide a blocking element which responds when a predetermined liquid or water level is reached in the tank, and causes the connection between the motor/blower area and the tank to be sealed off mechanically and, thus, to be interrupted. A known design of such a blocking element comprises a table-tennis ball which is guided in a cage and which is raised as the liquid level in the tank rises until it finally gets into the active suction area of the blower whereby it is drawn into contact with a seat which seals off the connection between the tank and the motor/blower.
In the case of this arrangement, the motor of the vacuum cleaner continues to operate and to drive the blower. In fact, this is of vital importance for the blocking effect as otherwise the table-tennis ball would loose its valve function and drop off. However, it is a problem of this arrangement that the person operating such a vacuum cleaner will not necessarily become aware of the fact that the connection has been interrupted as the overall noise produced by the unit will not change very much, while on the other hand the electric motor, being forced to work against a considerably higher resistance, may assume a dangerous condition. In any case, it may get considerably overheated, the windings may burn through and/or excessive carbon consumption, generation of sparks, or the like, may be encountered due to the electric motor operating against a high resistance, whereby its rotation is braked.
Now, it is the object of the present invention to improve a liquid or water vacuum cleaner, or combined dust and water vacuum cleaners, in such a manner that when the connection between the tank and the motor/blower area has been closed the motor will be protected and trouble or damage will be prevented.